1. 2.2 Write Definitions as Conditional Statements

2. Conditional Statement
A logical statement with a hypothesis and a conclusion.
If-then form: “If” part contains hypothesis
“then” part contains conclusion
Example: If it has four legs, then it is a dog.

3. Rewriting a Conditional Statement in if-then form
All whales are mammals.
Three points are collinear if there is a line containing them.

4. EXAMPLE 1
Rewrite a statement in if-then form
Rewrite the conditional statement in if-then form.
All birds have feathers. a. b.
Two angles are supplementary if they are a linear pair.
SOLUTION
First, identify the hypothesis and the conclusion. When you rewrite the statement in if-then form, you may need to reword the hypothesis or conclusion. a.
All birds have feathers.
If an animal is a bird, then it has feathers.

5. EXAMPLE 1
Rewrite a statement in if-then form b.
Two angles are supplementary if they are a linear pair.
If two angles are a linear pair, then they are supplementary.

6. GUIDED PRACTICE
for Example 1
Rewrite the conditional statement in if-then form. 1.
All 90° angles are right angles.
ANSWER
If the measure of an angle is 90°, then it is a right angle 2.
2x + 7 = 1, because x = –3
ANSWER
If x = –3, then 2x + 7 = 1

7. GUIDED PRACTICE
for Example 1
Rewrite the conditional statement in if-then form. 3.
When n = 9, n2 = 81.
ANSWER
If n = 9, then n2 = 81. 4.
Tourists at the Alamo are in Texas.
ANSWER
If tourists are at the Alamo, then they are in Texas.

8. Negation The opposite of the original statement
Original Statement: The ball is red
Negation: The ball is not red

9. Proving Conditional Statements
To show that a conditional statement is true, must show that the conclusion is true every time the hypothesis is true.
To show it’s false, give one counterexample.

10. Lingo Converse: Exchanges the hypothesis and conclusion
“The converse is the reverse”
Inverse: negate the hypothesis and conclusion
Contrapositive: Negates the converse

11. EXAMPLE 2
Write four related conditional statements
Write the if-then form, the converse, the inverse, and the contrapositive of the conditional statement “Guitar players are musicians.” Decide whether each statement is true or false.
SOLUTION
If-then form: If you are a guitar player, then you are a musician.
True, guitars players are musicians.
Converse: If you are a musician, then you are a guitar player.
False, not all musicians play the guitar.

12. EXAMPLE 2
Write four related conditional statements
Inverse: If you are not a guitar player, then you are not a musician.
False, even if you don’t play a guitar, you can still be
a musician.
Contrapositive: If you are not a musician, then you are not a guitar player.
True, a person who is not a musician cannot be a guitar player.

13. GUIDED PRACTICE
for Example 2
Write the converse, the inverse, and the contrapositive of the conditional statement. Tell whether each statement is true or false. 5.
If a dog is a Great Dane, then it is large
Converse: If the dog is large, then it is a Great Dane, False
Inverse: If dog is not a Great Dane, then it is not large, False
ANSWER
Contrapositive: If a dog is not large, then it is not a
Great Dane, True

14. GUIDED PRACTICE
for Example 2 6.
If a polygon is equilateral, then the polygon is regular.
Converse: If polygon is regular, then it is equilateral, True
Inverse: If a polygon is not equilateral, then it is not regular, True
Contrapositive: If a polygon is not regular, then it is not equilateral, False
ANSWER

15. Working with definitions
Any definition can be made into an if-then statement
On page 81: Perpendicular lines are two lines that intersect to form a right angle.
If-then: If two lines intersect at a right angle, then they are perpendicular lines.

16. Reminders
How do you tell angles are congruent by looking at a diagram?
How do you know an angle is a right angle?
How do you know segments are congruent?

17. EXAMPLE 3
Use definitions
Decide whether each statement about the diagram is true. Explain your answer using the definitions you have learned. a.
AC BD b.
AEB and CEB are a linear pair. c.
EA and EB are opposite rays.

18. EXAMPLE 3
Use definitions
SOLUTION a.
This statement is true. The right angle symbol in the diagram indicates that the lines intersect to form a right angle. So you can say the lines are perpendicular. b.
This statement is true. By definition, if the noncommon sides of adjacent angles are opposite rays, then the angles are a linear pair. Because EA and EC are opposite rays, AEB and CEB are a linear pair.

19. EXAMPLE 3
Use definitions c.
This statement is false. Point E does not lie on the same line as A and B, so the rays are not opposite rays.

20. GUIDED PRACTICE
for Example 3
Use the diagram shown. Decide whether each statement is true. Explain your answer using the definitions you have learned. 7.
JMF and FMG are supplementary.
This statement is true because linear pairs of angles are supplementary.
ANSWER

21. GUIDED PRACTICE
for Example 3
Use the diagram shown. Decide whether each statement is true. Explain your answer using the definitions you have learned. 8.
Point M is the midpoint of FH .
ANSWER
This statement is false because it is not known that FM = MH . So, all you can say is that M is a point of FH

22. GUIDED PRACTICE
for Example 3
Use the diagram shown. Decide whether each statement is true. Explain your answer using the definitions you have learned. 9.
JMF and HMG are vertical angles.
ANSWER
This statement is true because in the diagram two intersecting lines form 2 pairs of vertical angles.

23. GUIDED PRACTICE
for Example 3
Use the diagram shown. Decide whether each statement is true. Explain your answer using the definitions you have learned.
10.
FH JG
ANSWER
This statement is false : By definition if two intersect to form a right angle then they are perpendicular. But in the diagram it is not known that the lines intersect at right angles . So you cannot say that
FH JG

24. Biconditional Statement
Contains the words “if and only if”
In order to be a true biconditional, it has to be a true if-then conditional statement, and have a true converse

25. EXAMPLE 4
Write a biconditional
Write the definition of perpendicular lines as a biconditional.
SOLUTION
Definition: If two lines intersect to form a right angle, then they are perpendicular.
Converse: If two lines are perpendicular, then they intersect to form a right angle.
Biconditional: Two lines are perpendicular if and only if they intersect to form a right angle.

26. GUIDED PRACTICE
for Example 4
11.
Rewrite the definition of right angle as a biconditional statement.
Biconditional:
An angle is a right angle if and only if the measure of the angle is 90°
ANSWER

27. GUIDED PRACTICE
for Example 4
12.
Rewrite the statements as a biconditional.
If Mary is in theater class, she will be in the fall play. If Mary is in the fall play, she must be taking theater class.
Biconditional:
Mary is in the theater class if and only if she will be in the fall play.
ANSWER

28. Homework
1-21, 31, 33-35